The following description of the background of the invention is provided simply to aid in understanding the invention and is not admitted to describe or constitute prior art to the invention.
Apolipoprotein A-I (apoA-I) is the major protein component of the High-Density Lipoprotein (HDL) blood fraction having a nominal concentration of about 1.0 to 1.5 mg/mL in plasma. Mature apoA-I is a single polypeptide chain of 243 amino acids, which maps in humans to gene locus 11q23. ApoA-I is synthesized in the liver and small intestine as a 267-residue preproapolipoprotein. An 18-amino acid residue presequence is cleaved during translation by a signal peptidase, and the resulting proapoA-I contains a hexapeptide prosegment covalently linked to the amino terminus of mature apoA-I. ProapoA-I is secreted into the plasma and lymph and undergoes extracellular posttranslational cleavage of the hexapeptide prosegment to give rise to the mature 243-residue apoA-I.
Multiple properties and functions are ascribed to apoA-I. Specifically, apoA-I is a cofactor for lecithin cholesterol acyltransferase (LCAT), which is responsible for the conversion of cholesterol and phosphatidylcholines (lecithins) to cholesteryl esters and lysophosphatidylcholines on the surface of high density lipoproteins. Hence, apoA-I is implicated in cholesterol metabolism. ApoA-I, as a key element of the reverse cholesterol transport pathway, promotes efflux of cholesterol from cells which include vessel walls. Hence, apoA-I is envisaged to provide a protective mechanism against the development of atherosclerosis. Furthermore, apoA-I is implicated in the organization of HDL structure.
A correlation between the occurrence of cardiovascular disease (CVD), and in particular atherosclerotic heart disease, and oxidative processes is long known in the art. See e.g., Chisolm & Steinberg (2000) Free Radic. Biol. Med. 28:1815-1826; Navab et al. (2002) Curr. Opin. Lipidol. 13:363-372; Ross (2003) New Engl. J. Med. 340:115-126; and Heinecke (2003) Am. J. Cardiol. 91:12A-16A. For example, the extent of modification of apoA-I by oxidation of tyrosine (i.e., chlorination and nitration) is correlated with functional impairment in reverse cholesterol transport activity of isolated apoA-I [Zheng et al. (2004) J. Clin. Investig. 114:529-541.]
Further investigation (e.g., Zheng et al. (2004) J. Clin. Investig. 114:529-541) has demonstrated that apoA-I is a selective target for nitration and chlorination in vivo by myeloperoxidase (MPO, EC 1.11.1.7). MPO is a heme-containing protein that maps to human gene locus 17q23.1, which is synthesized by various phagocytes. Although produced as a single chain precursor of 80-kD, MPO is subsequently cleaved into light (12-kD) and heavy (60-kD) chains, which form mature MPO, a tetramer composed of 2 light chains and 2 heavy chains. MPO utilizes hydrogen peroxide and chloride to generate chlorinating oxidants, e.g., HOCl, that kill pathogens. Additionally, in human monocytes, MPO utilizes hydrogen peroxide and nitrite to generate nitrating oxidants capable of lipid and protein nitration. A correlation has been established between plasma MPO levels and the incidence of myocardial infarction in patients presenting with chest pain (Brennan et al. (2003) New Engl. J. Med. 349:1595-1604).
Further elucidation of the role of MPO-mediated oxidation of apoA-I has been provided by various investigators (e.g., Zheng et al., J. Biol. Chem. (2005) 280:38-47) which identified the sites of MPO-mediated oxidation of apoA-I in vitro and in vivo as Tyr192, Tyr166, Tyr236, and Tyr29 (SEQ ID NO: 2). Site-specific quantitative analyses conducted by these investigators demonstrate that the favored modification site within apoA-I after exposure to MPO-generated oxidants is Tyr192, with Tyr166 oxidized as a secondary site, and with Tyr236 and Tyr29 modified only minimally. The term “favored modification site” refers to the apoA-I amino acid which is oxidized to the greatest extent after exposure to MPO-generated oxidants. The term “secondary site” refers to an apoA-I amino acid which may be oxidized after exposure to MPO-generated oxidants, but for which the frequency of oxidation is not as great as for a favored modification site. The term “minimally” in this context refers to amino acid sites of apoA-I which undergo a low frequency of oxidation by MPO-generated oxidants under physiological conditions.